In plastic working typified by forging, wire drawing, tube drawing, roll forming, pressing and the like, friction that is caused when metal surfaces (especially, of dies and works) intensively rub with each other may cause an increase in working energy, heat generation, seizure phenomenon and the like, for which various lubricants have been used with intent to reduce the frictional force. Traditionally, oils and soaps have been used as lubricants and fed to frictional surfaces as fluid lubricating films to reduce the frictional force. However, they involve large heat generation due to enlargement of surface area and are insufficient in lubricity for plastic working in which sliding occurs under high contact pressure, allowing seizure to more easily occur due to a break in the lubricating films and so on. Therefor, a technique has been generalized and widely used, in which metallic material surfaces are coated in advance with inorganic films such as borax films and crystallized phosphate films or solid films such as resin films with sufficient film strength, which intervene at interfaces between dies and works even under high contact pressure so that a break in the lubricating films may not easily occur and the direct contact between metals may be avoided.
On the other hand, requirements for solid films are widely ranging and being rapidly increased in recent years, including a further reduction of working energy, an increase in intense working, adaptation to refractory works, environmental friendliness of filming processes (for example, phosphating produces a large amount of industrial waste such as sludge, causing problems in environmental conservation) and adaptation to lubricating powder-free or oil-free working. To address these requirements while considering environmental conservation, solid films having a high degree of lubricity are being developed. The technique forms films having a high degree of lubricity through a convenient step of applying a water-based lubricant for plastic working to surfaces of works and drying the lubricant. As such a technique, a lubricant composition for plastic working of metallic materials, containing (A) a synthetic resin, (B) a water-soluble inorganic salt and water, wherein (B)/(A) (mass ratio of solid content) is from 0.25/1 to 9/1 and the synthetic resin is dissolved or dispersed is disclosed in Patent Reference 1. It is also described in Patent Reference 1 that it is preferred to incorporate 1 to 20% by mass of at least one selected from the group consisting of a metallic soap, a wax, polytetrafluoroethylene and an oil as a lubricating component and that at least one selected from the group consisting of a sulfate, a borate, a molybdate, a vanadate and a tungstate is preferred as the water-soluble inorganic salt. This technique is an excellent one in which a lubricating film is formed of lubricating components such as a soap and wax being bound in a solid film consisting of (A) the synthetic resin and (B) the water-soluble inorganic salt and the lubricating film is coated on a surface of a work to obtain a lubricating film having a high degree of workability in a convenient and power-saving manner. This technique is widely used mainly in the field of plastic working and is a promising technique because techniques excellent even in intense working applications, which have a greater surface area extension in comparison with the combination of industrially established phosphate films and soaps, are being developed.    Patent Reference 1: Japanese Patent No. 3881129
Also, a water-based lubricant for plastic working of metallic materials as a composition containing (A) at least one water-soluble inorganic salt selected from the group consisting of a sulfate, a silicate, a borate, a molybdate and a tungstate and (B) a wax, dissolved or dispersed in water optionally with a surface active agent, wherein the mass ratio of solid content (B)/(A) is within the range of 0.3 to 1.5 is disclosed in Patent Reference 2. This technique is an excellent one in which a water-soluble inorganic salt is used as a principal component of a solid film and a lubricating wax is incorporated in the solid film to provide a high degree of working performance, similarly to Patent Reference 1.    Patent Reference 2: Japanese Patent No. 3984159
As stated in Patent References 1 and 2, water-soluble inorganic salts and water-soluble resins are essential components of solid films of water-based lubricants for plastic working, because lubricating films composed of water-soluble inorganic salts and/or water-soluble resins have sufficient film strength and, as mentioned above, intervene at interfaces between dies and works even under high contact pressure so that a break in the lubricating films may not easily occur and direct contact between metals may be avoided. For water-based lubricants for plastic working, therefore, the combination of solid films composed of water-soluble inorganic salts and/or water-soluble resins with appropriate slip additives capable of reducing coefficient of friction allows to maintain good lubricating conditions during plastic working.
The mechanism of film forming of water-based lubricants for plastic working composed of water-soluble components will be described. Water-soluble inorganic salts and water-soluble resins as the water-soluble components are dissolved or dispersed in water in lubricant treatment liquids and, when the lubricants are applied to metallic material surfaces followed by drying, water as solvent will evaporate so that lubricating films may be formed. In the meantime, the water-soluble inorganic salts and the water-soluble resins will deposit as solids on the metallic material surfaces to form solid films. The solid films thus formed possess sufficient film strength to withstand plastic working and, with suitable slip additives incorporated for reducing coefficient of friction, exhibit good lubricity during plastic working.
However, the water-soluble components have deliquescency and/or hygroscopicity because of water solubility, and therefore, the solid films formed over the metallic material surfaces will absorb moisture by absorbing water vapor in the atmosphere under high-temperature/high-humidity environments. Through moisture absorption, the solid films will be swollen with or dissolved in water, gradually turning from solid to fluid. When the solid films fluidize, the film strength will markedly decrease, causing a break in the lubricating films at the interface between dies and works under high contact pressure during plastic working and allowing direct contact between metals to occur. Therefore, lubricants for plastic working whose solid films are composed of water-soluble components such as water-soluble inorganic salts and water-soluble resins absorb moisture under high-temperature/high-humidity environments to greatly reduce their lubricity, workability and seizure resistance.
Also, since water-soluble components absorb water, which can be a corrosion medium for metals, through moisture absorption, rust will be produced on metallic material surfaces. When rust is produced, it will not only deteriorate the appearance but also degrade the dimensional accuracy at worked surfaces. In plastic working, it is important that a metallic material is shaped exactly to the shape of a die when pressed, with qualities higher when dimensions are more accurate and forged surface textures are smoother. Therefore, rust produced before press working increases frictional force to thereby reduce lubricity, leading to the degradation of dimensional accuracy and/or the deterioration of forged surface textures through the indentation of the rust at the worked surfaces. Also, rust produced after press working increases the surface roughness at worked surfaces, leading to the degradation of dimensional accuracy and the deterioration of forged surface textures.
As mentioned above, lubricating films composed of water-soluble components absorb moisture under high-temperature/high-humidity environments to cause the degradation of lubricating performance and rusting. Therefore, it is difficult to store metallic materials over which lubricating films are formed in exposure to the atmosphere for an extended period of time. If a lubricated metallic material was placed in a hermetically sealed container with a moisture-proof agent introduced to suppress moisture absorption, storage for an extended period of time would be possible; at production sites, however, mass production and mass storage are made in most cases, and such a method of storage would be industrially impractical.
On the other hand, in phosphating typified by bonderizing, chemical conversion reaction occurs on the surface of a work to deposit a crystalline phosphate. A phosphate is water-insoluble and will not absorb moisture even under high-temperature/high-humidity environments. Therefore, the lubricating performance will not degrade and, with excellent corrosion resistance, the degradation of dimensional accuracy or the deterioration of forged surface textures due to rusting will not occur. Therefore, storage even under high-temperature/high-humidity environments for an extended period of time is possible, without concern of the effects of moisture absorption and rusting. However, phosphating produces a large amount of industrial waste such as sludge in film treatment, causing problems in environmental conservation.
Also, in-line systems have been put into practical use, which continuously carry out the steps from formation of a lubricating film to press working, as a countermeasure for preventing moisture absorption. According to this method, since press working is made before moisture absorption, the effects of moisture absorption on a lubricating film may be ignored, and simultaneously, productivity can conveniently be improved. According to such systems, however, the lubricating film will absorb moisture in cases where, for example, an extended period of line shutdown occurs due to some necessity in production such as troubles and/or maintenance. When the lubricating film has a film temperature higher than the outside air temperature due to the preheating at a drying step, the moisture in the film will tend to evaporate and no moisture absorption will occur, but when the temperature drops to the outside air temperature, moisture absorption will start. In any case, the moisture absorption by the lubricating film may not be avoided under the environment where the film temperature drops to the outside air temperature.
Water-soluble inorganic salts and synthetic resins are generally used in solid films of water-based lubricants for plastic working and, among the wide variety of such synthetic resins, there are components that are less susceptible to moisture absorption in comparison with the water-soluble inorganic salts. Specifically, among the synthetic resins described in Patent Reference 1, acrylic resins, vinyl acetate resins, epoxy resins, urethane resins and phenolic resins may be mentioned. These synthetic resins have less hydrophilic groups responsible for moisture absorption in their structures, with less affinity with water, and therefore, are excellent in water resistance and less susceptible to performance degradation due to moisture absorption. However, these synthetic resins are dispersed as particles in the water-based lubricants and, when the water-based lubricants are heated in use for the purpose of accelerating drying of the lubricating films, the particles will flocculate each other to immediately deteriorate the dispersed state. Since the water-soluble inorganic salts exist as ions in the water-based lubricants, they can be used as heated at below 80° C. with no problems in liquid stability. Therefore, the synthetic resins are inferior in dispersion stability in the water-based lubricants to the water-soluble inorganic salts.
Furthermore, while such synthetic resins are excellent in water resistance, they are poor in film removal, causing various failures during subsequent steps. It is a concern that, if film removal is insufficient, for example, film components may contaminate a cutting coolant during a cutting step after the press working and, for a case of gear parts, they may contaminate a lubricating oil after assembly into transport equipment. Also, when plating is carried out after a film removal step, film components may not only contaminate the plating solution but also cause plating failures at portion where the film components remain.
For film removal, a lubricant composition for forming lubricating films removable by water rinsing, with use of a synthetic resin excellent in film removal properties as a solid film, is described in Patent Reference 3. This technique is a lubricant composition for forming lubricating films removable by water rinsing, comprising (a) at least one selected from water-soluble polyesters having an average molecular weight of 30,000 to less than 500,000 and water-soluble polysaccharides, (b) at least one selected from water-soluble polyamides, (c) at least one selected from waxes having a melting point of 50 to 130° C. and (d) water, wherein the weight ratio of (a)/(b) is 50/1 to 1/50, and the content of (c) is 3 to 90 parts by weight based on 100 parts by weight of the sum of (a) and (b). However, the solid film of this lubricant is mainly based on a synthetic resin, with no incorporated components for improving film strength such as water-soluble inorganic salts. Therefore, it does not have sufficient film strength for plastic working, allowing a break in the film under high contact pressure and causing seizure with dies. Such a lubricant is therefore insufficient in performance under stringent working conditions.    Patent Reference 3: Japanese Patent No. 3285962
Therefore, water-based lubricants for plastic working composed of water-soluble components, with which no degradation of lubricity or seizure resistance will occur by moisture absorption even under high-temperature/high-humidity environments and which are excellent in corrosion resistance so that the degradation of dimensional accuracy or the deterioration of forged surface textures due to rusting on the worked surfaces may not occur, have not yet been obtained. In addition, water-based lubricants that can be used while heated and provide easy film removal have not yet been obtained.
In another aspect, since the water-based lubricants for plastic working in Patent References 1 and 2 have high affinity with water and low water resistance, they will allow, under high-temperature/high-humidity environments, water vapor in the atmosphere to infiltrate into lubricating films and reach metallic material surfaces to produce rust. When rust is produced, it will not only deteriorate the appearance but also degrade the dimensional accuracy at worked surfaces. In plastic working, it is important that a metallic material is shaped exactly to the shape of a die when pressed, with qualities higher when dimensions are more accurate and forged surface textures are smoother. Therefore, rust produced before press working increases frictional force to thereby reduce lubricity, leading to the degradation of dimensional accuracy and/or the deterioration of forged surface textures through the indentation of the rust at the worked surfaces. Also, rust produced after press working increases the surface roughness at the worked surfaces, leading to the degradation of dimensional accuracy and the deterioration of forged surface textures.
On the other hand, in phosphating typified by bonderizing, chemical conversion reaction occurs on the surface of a work to deposit a crystalline phosphate. A phosphate is water-insoluble and has high water resistance, and therefore, is excellent in corrosion resistance, so that the degradation of dimensional accuracy or the deterioration of forged surface textures due to rusting will not occur. Therefore, storage even under high-temperature/high-humidity environments for an extended period of time is possible, without concern of the effects of rusting. However, phosphating produces a large amount of industrial waste such as sludge in film treatment, causing problems in environmental conservation.
Water-soluble inorganic salts and synthetic resins are generally used in solid films of water-based, lubricating film treatment agents for plastic working and, among the wide variety of such synthetic resins, there are components that are more water-resistant than the water-soluble inorganic salts. Specifically, among the resins described in Patent Reference 1, acrylic resins, vinyl acetate resins, epoxy resins, urethane resins and phenolic resins may be mentioned. These resins have less hydrophilic groups in their structures, with less affinity with water, and therefore, are high in water resistance and exhibit excellent corrosion resistance. When plastic forming is carried out, however, these synthetic resins are poor in conformability to metallic material surfaces during material deformation, reducing remaining films with the result that sufficient corrosion resistance may not be obtained.
Therefore, water-based lubricants for plastic working composed of water-soluble inorganic salts or synthetic resins as principal components, which are excellent in corrosion resistance under high-temperature/high-humidity environments and with which no degradation of dimensional accuracy or no deterioration of forged surface textures due to rusting at worked surfaces may occur, have not yet been obtained.